The present invention relates generally to hitch structure for connecting an implement to a towing vehicle, and more specifically, to three-point hitch structure which permits a trailing implement to follow the ground contour.
When operating a farm machine over uneven terrain, such as terraces or ditches, there may be a problem with the machine maintaining proper relationship with the slope or contour of the soil. This is especially true, for example, when the machine includes a unit carrier towed by a tractor or other vehicle with an implement connected rearwardly of the unit carrier. If the towing implement is a tillage or planting unit, proper relationship of the unit to the soil is very important for good soil working and planting characteristics. Commonly, the trailing implement is connected to the unit carrier by a three-point hitch arrangement which generally limits the freedom of the implement to move in relation to the unit carrier. Although the problem may not be as acute when a trailing implement is mounted directly to a towing vehicle such as a tractor by a three-point hitch arrangement, the hitch does tend to limit the ability of the trailing implement to follow the ground contour and often the position of the forward portion of the implement is determined by the wheels of the towing vehicle. The trailing implement, which may be a grain drill or the like, is attached to the tractor or the unit carrier using two lower draft links and one upper rigid stabilizing link. When in the operating position, the implement is supported on a set of wheels, such as gauge wheels or press wheels, with its attitude controlled by the upper stabilizing link. As the implement passes over uneven terrain, the attitude of the implement will be changed which adversely affects the relationship of the planting unit or other implement to the soil. Although flexible upper link arrangements such as exemplified in U.S. Pat. No. 4,232,878 have been suggested, they have not been entirely satisfactory because the lost motion in the link causes the trailing implement to angle downwardly in the rearward direction when the hitch is raised to the transport position, thus limiting the rearward clearance of the implement as it is being towed. In addition, the flexibility of the implement may be limited to less than a desirable range and gauging of the forward portion of the implement is dependant on the rear tractor wheels which are usually offset a substantial distance ahead of the implement. In some hitch arrangements, the upper link consists of a hydraulic cylinder, but to provide freedom for the trailing machine to move over ground contours requires that the cylinder be free-floating during field working operations and necessitates special valving or a separate hydraulic circuit with separate controls. A hitch with such a link, therefore, is often more costly and complex and less convenient to operate than other types of three-point hitch arrangements.